Field of the Invention
The present invention relates generally to SiC coatings and more specifically to SiC coatings on carbon materials.
Description of the Prior Art
SiC is a hard, strong, chemically resistant material that is thermally and electrically conductive. Due to its covalent bonding, it is very difficult to densify without the use of additives that compromise its material properties. However, in many cases a monolithic material is not needed. SiC can be deposited as a coating by techniques such as chemical vapor deposition (CVD), plasma enhanced CVD, and direct chemical reaction between carbon and molten silicon. SiC coatings are used to protect a weaker or less resistant material by isolating it from the environment. The protection can be in the form of mechanical protection such as wear or abrasion resistance, or chemical protection in corrosive or reactive environments. Examples of mechanical protection are wear resistant coatings on bearings and wear plates. Examples of chemical protection are coated filters for molten metals and coatings for the chemical industry. An example of a combination of wear resistance and chemical resistance where SiC coating are used is in gas turbine engines.
CVD of SiC requires specialized equipment that can hold temperatures of 2000° C. at a pressure of 0.5 Torr+/−0.001 for up to 10 days. The growth process is relatively slow and expensive. The gasses used are methylsilane and trimethylsilane which are extremely flammable and considered hazardous.
The direct chemical method infiltrates carbon fleece with molten silicon at 1700° C. The carbon and Si is placed on the material to be coated and heated to 700° C. to bond. The substrate and carbon and silicon mix is heated to 1700° C. to react the Si and C to make SiC. The reaction is generally not complete and is composed of SiC and free Si. Since the reaction is a liquid phase reaction, the impurities are not volatilized as they are in a vapor phase reaction and result in a coating that is dependent on the purity of the silicon.